Astron. Astrophys. 364, 165-169 (2000)

6. Discussion

In the 1980's there was a large number of reports of VHE gamma ray emission from X-ray binaries made with first-generation non-imaging telescopes (for a review see e.g. Chadwick et al. 1990). Initial observations with imaging telescopes, especially in the northern hemisphere, failed to substantiate these detections and the evidence for VHE emission from X-ray binaries has been questioned (for a recent review see Hoffman et al. 1999).

However, recent observations of a number of X-ray binaries with the EGRET detector have shown the possibilty of high energy emission from these objects. A source consistent with the position of Cyg X-3 has been seen at good significance in the EGRET data but does not exhibit the characteristic 4.8 hr orbital modulation (Michelson et al. 1992; Mori et al. 1997). There is good evidence for emission from Cen X-3 (Vestrand et al. 1997) in the EGRET data. The enigmatic binary source LS I  has been suggested as an emitter at energies above 100 MeV (Kniffen et al. 1997) and above 1 GeV (Lamb & Macomb 1997). The putative X-ray binary SAX J0635+0533 has also been associated with the EGRET source 2EG J0635+0521 (Kaarat et al. 1999; Cusumano et al. 2000).

The lack of orbital modulation in our VHE data can be used to rule out a number of models of VHE emission. Moskalenko and co-workers have developed a model where the VHE gamma rays are produced close to the neutron star (Moskalenko et al. 1993; Moskalenko & Karakula 1994). A characteristic of such a model is that the VHE emission will exhibit an orbital modulation due to pair production by the gamma rays on the photon field around the companion star. When applied to Cen X-3, this model leads to a prediction of VHE emission confined to orbital phases between 0.35 and 0.65 at energies of GeV. Detection of VHE emission at orbital phases other than those predicted does not support this model.

Aharonian & Atoyan (1991, 1996) have discussed a model for VHE gamma ray production in X-ray binaries where a beam of relativistic particles interacts with a moving gas target which has been ejected by the companion star. This model was proposed to explain the episodic emission of pulsed VHE gamma rays seen in earlier observations. Although the model is optimised to produce pulsed gamma rays, the dimensions and density of the gas target are critical; non-optimal target sizes can cause the coherence to be lost. Thus the lack of a pulsed signal in these observations does not necessarily exclude models of this class. Although the model predicts natural timescales of h for the interaction of the particle beam with the gas cloud, the sensitivity of the present measurements is not enough to distinguish such episodes.

An interesting model for production of VHE emission in accreting X-ray binaries is that of Katz & Smith (Katz & Smith 1988; Smith et al. 1992). The model is based on proton acceleration due to turbulence in the accretion column which, in turn, produces fluctuations in the strength of the magnetic field which travels up the accretion column. This results in acceleration of protons to high energy via resonant absorption in the outer regions of the magnetosphere. As well as predicting the existence of VHE gamma rays, this model also predicts that the protons will emit synchrotron radiation up to GeV, that there will be detectable fluctuations in the X-ray signal on millsecond timescales due to turbulence in the accretion column and the possibility of the synchrotron radiation being detectable in the infrared. The observations of GeV emission from Cen X-3 (Vestrand et al. 1997) and kHz fluctuations in the X-ray intensity (Jernigan et al. 2000) indicate that such a model for VHE production should be re-evaluated. We also note that the epoch at which kHz fluctuations were observed (JD 2 450 507 - JD 2 450 512, Jernigan et al. 2000) overlaps with with our most significant and strongest VHE detection (JD 2 450 508 - JD 2 450 511).

© European Southern Observatory (ESO) 2000

Online publication: December 15, 2000
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